A uniform gas flow is often desired across the surface of a device being processed. In many instances a uniform gas flow over an entire surface of a device is difficult to achieve because of, e.g., structural irregularities or asymmetries, such as an off center pump port, in the chamber of the gas flow system in which the device is contained. In the semiconductor technology, a uniform gas flow is required in a system for processing a semiconductor wafer at the surface of the wafer even though the chamber of the system containing the wafer includes asymmetrically arranged elements therein located between the gas inlet and the gas outlet which cause disturbances in the flow of the gas in the chamber. In many commonly used semiconductor wafer processing systems, the chamber is designed so that even though a processing gas is introduced uniformly into the chamber and directed at the surface of a wafer, disturbances are created by an asymmetrically positioned exhaust pump port or outlet or by other required structure in the chamber.
In view of the very high density of elements, such as transistors, formed in today's semiconductor wafers, which now includes more than 16 million memory cells in a single semiconductor chip diced from a wafer, it can be seen that a highly uniform gas flow is required over the surface of the wafer, e.g., in an etching process, in order to obtain satisfactory processing yields. This uniform gas flow is particularly important when a uniform layer, e.g., a polysilicon layer having a thickness of 2,000 Angstroms or less, deposited on the wafer must be selectively etched across the entire surface of the wafer without disturbing an underlying thin layer, e.g., a layer of silicon nitride, to produce the many devices formed in the wafer with the same electrical characteristics, particularly since today's semiconductor wafers may have an eight inch or larger diameter.
In U.S. Pat. No. 4,780,169 by M. M. Stark et al, which issued on Oct. 25, 1988, there is disclosed a reaction chamber of a dry etching apparatus including a gas inlet having a non-uniform array of inlets for a non-uniform introduction of an etching gas to compensate for non-uniformity characteristics of the apparatus to provide a uniform etch in a substantially symmetrical chamber. Also, U.S. Pat. No. 4,297,162 by R. S. Mundt et al, which issued on Oct. 27, 1981, discloses the use of a curved electrode in a symmetrical chamber through which reactant gases pass to a semiconductor slice, the curved electrode being closer to the slice at its center than at its periphery to improve etching on the slice.
In order to enhance the uniformity of laminated film etching in plasma processing apparatus, there is disclosed in U.S. Pat. No. 5,246,532 by T. Ishida, which issued on Sep. 21, 1993, a focus ring surrounding a substrate to be processed, the height of the focus ring relative to the substrate being controlled by an electromagnet.
U.S. Pat. No. 5,271,788 by M. Hasegawa et al, which issued on Dec. 21, 1993, discloses magnetron plasma etching apparatus for improving uniformity of etching of a wafer including a low resistance carbon ring having an outer diameter larger than the diameter of the wafer disposed on a susceptor of the apparatus, the low resistance carbon ring being electrically connected to the susceptor.
U.S. Pat. No. 4,512,841 by G. F. Cunningham et al, issued on Apr. 23, 1985, discloses a batch type, parallel plasma etcher for providing improved etching by compensating for the plasma field or chamber abnormalities by using different wafer holders suitable for insertion in a common electrode of the etcher.